Well Concrete has traditionally be used in power plants because it is very cheap in materials terms, however it now appears that materials costs are not a large fraction of the cost of the plant.
The real costs appear to be in construction step, and concrete is time consuming and expensive to work with, with rebar fabrication, vibration and curing.

It appears likely that cellular steel structures that can be bolted together could replace them, with randomly packed lead shot for gamma shielding, without significantly increasing the materials construction cost.

We could literally make a reactor building roof out of a dozen or more 'slats' for example.
And if we used steel vibration or screw piles we don't even need concrete foundations.
Just a thought

E Ireland wrote:Problem with using water - beyond the bulk - is that you would have to account for what happens if you actually evaporate the water, or it leaks.

In an accident you could be faced with having to abandon parts of the reactor building, even if the fuel hasn't actually failed yet.

This would depend on the amount of water; if it is large enough to not boil during the worst accident then it obviously isn't an issue - and would improve safety by having a huge and totally passive heat sink.

Bulk is not of much concern for a stationary powerplant. It is an issue of course with any mobile app.

I am thinking cellular steel for structural rigidity, filled with either boron carbide or lead balls for shielding, using sufficiently conservative estimates that random packing can be used as a packing method.

Suppose you have a heavy reactor vessel made of steel, but you would like to have some shielding material that would cause some pressure around the reactor vessel, and was also quite easily removed when needed. Some kind of heavy sand (limenite or other) could be useful. Sands may be easily removed simply by gravity assisted flow like in a hourglass, and if some amount of sand was left in pockets, a vacuum cleaner could remove the remaining sand. A supporting structure holding sand would be needed, and that could be a steel construction. Sand between these solid components could be recycled. Sand could provide multiple functions: radiation shielding, thermal insulation, even pressure to support reactor vessel, and vibration dampening in case of an earthquake.

Transuranic heavy elements may not be used where there is life.
– Sapphire & Steel intro